The version number of the configuration is stored in the mp3tag.cfg file. So if you copy both an old pre-v2.89 mp3tag.cfg and usrfields.ini the migration will be performed.
If you only copy an old pre-v2.89 usrfields.ini, there won't be any migration because Mp3tag assumes to already start from the latest version.
Have you copied both mp3tag.cfg and usrfields.ini to your new installation while Mp3tag being closed? If so, you could send me the zip file and I'll further analyze the issue.
I'm pretty sure I just used the Save configuration feature to create a zip file and when it came to restoring it, I copied everything from that zip to %APPDATA%\Roaming\Mp3tag. I must have, because my custom actions were there. I can send you the zip file, if that helps.
I've tried the process with the configuration you've sent and the Tag Panel migration worked without problems. It also shows the default fields above the directory field.
I just tried again too - quit the application, deleted the %APPDATA%\Roaming\Mp3tag folder, restarted the application (tag panel as expected), quit the application, copied over the files from the zip, started, tag panel was as expected and custom actions there. Not sure how it got in the previous state. Thanks for looking!
I probably didn't copy the mp3tag.cfg from the backup zip when restoring and only chose the data and export directories.